An internal combustion engine of the present description may be used as a motor vehicle drive unit. Within the context of the present disclosure, the expression “internal combustion engine” encompasses diesel engines and Otto-cycle engines, but also hybrid internal combustion engines, that is to say internal combustion engines which are operated with a hybrid combustion process, and hybrid drives which, in addition to the internal combustion engine, comprise at least one further torque source for driving a motor vehicle, for example an electric machine which is connectable in terms of drive to the internal combustion engine and which outputs power instead of the internal combustion engine or in addition to the internal combustion engine.
In the development of internal combustion engines, it may be constantly sought to minimize fuel consumption. Furthermore, a reduction of the pollutant emissions is sought in order to be able to comply with future limit values for pollutant emissions.
Internal combustion engines may be equipped with a supercharging arrangement, wherein supercharging is associated with a method for increasing power, in which the charge air used for the combustion process in the engine is compressed, as a result of which a greater mass of charge air can be supplied to each cylinder per working cycle. In this way, the fuel mass and therefore the mean pressure can be increased.
Supercharging may be a suitable method for increasing the power of an internal combustion engine while maintaining an unchanged swept volume, or for reducing the swept volume while maintaining the same power. In most cases, supercharging may lead to an increase in volumetric power output and a more expedient power-to-weight ratio. If the swept volume is reduced, it is possible, given the same vehicle boundary conditions, to shift the load collective toward higher loads, at which the specific fuel consumption is lower. Supercharging of an internal combustion engine may serendipitously assist in the efforts to minimize fuel consumption, that is to say to improve the efficiency of the internal combustion engine.
A suitable transmission configuration may additionally allow downspeeding, whereby a lower specific fuel consumption is likewise achieved. In the case of downspeeding, use is made of the fact that the specific fuel consumption at low engine speeds is generally lower, in particular in the presence of relatively high loads.
With targeted configuration of the supercharging, it is also possible to obtain advantages with regard to the exhaust-gas emissions. With suitable supercharging for example of a diesel engine, the nitrogen oxide emissions can therefore be reduced without any losses in efficiency. At the same time, the hydrocarbon emissions can be positively influenced. The emissions of carbon dioxide, which correlate directly with fuel consumption, likewise decrease with falling fuel consumption.
To comply with future limit values for pollutant emissions, however, further measures may be desired. One example may include nitrogen oxides, wherein the reduction of nitrogen oxide emissions, which are of high relevance in particular in diesel engines. Since the formation of nitrogen oxides occurs with not only an excess of air but also high temperatures, one concept for lowering the nitrogen oxide emissions consists of using combustion processes with lower combustion temperatures.
Here, exhaust-gas recirculation (EGR), that is to say the recirculation of combustion gases from the outlet side (e.g., exhaust system) to the inlet side (e.g., intake system), may be desired in achieving this aim, wherein it is possible for the nitrogen oxide emissions to be reduced by increasing exhaust-gas recirculation rate. Here, the exhaust-gas recirculation rate xEGR is determined as xEGR=mEGR/(mEGR+mfresh air), where mEGR denotes the mass of recirculated exhaust gas and mfresh air denotes the supplied fresh air. The oxygen provided via exhaust-gas recirculation may be taken into consideration.
To obtain a considerable reduction in nitrogen oxide emissions, high exhaust-gas recirculation rates may be used, which may be of the order of magnitude of xEGR≈60% to 70% or more. Such high recirculation rates may demand cooling of the exhaust gas for recirculation, by which the temperature of the exhaust gas may be reduced and the density of the exhaust gas increased, so that a greater mass of exhaust gas can be recirculated. Consequently, an exhaust-gas recirculation arrangement may be equipped with a cooler. The exhaust-gas recirculation arrangement of the internal combustion engine to which the present disclosure relates comprises a cooling arrangement, that is to say at least one EGR cooler, which has a coolant-conducting coolant jacket which serves for the transfer of heat between exhaust gas and coolant.
Problems can arise during the introduction of the recirculated exhaust gas into the intake system if the temperature of the recirculated hot exhaust gas decreases and condensate forms. Firstly, condensate may form if the recirculated hot exhaust gas meets, and is mixed with, cool fresh air in the intake system. The exhaust gas cools down, whereas the temperature of the fresh air is increased. The temperature of the mixture of fresh air and recirculated exhaust gas, that is to say the temperature of the combustion air, lies below the exhaust-gas temperature of the recirculated exhaust gas. During the course of the cooling of the exhaust gas, liquids previously contained in the exhaust gas and/or in the combustion air still in gaseous form, in particular water, may condense out if the dew point temperature of a component of the gaseous combustion-air flow is undershot. Condensate formation occurs in the free combustion-air flow, wherein contaminants in the combustion air often form the starting point for the formation of condensate droplets.
Secondly, condensate may form when the recirculated hot exhaust gas and/or the combustion air impinges on the internal wall of the intake system, as the wall temperature may lie below the dew point temperature of the relevant gaseous components.
Condensate and condensate droplets may be undesirable and lead to increased noise emission in the intake system and may collide with the impeller blades of a compressor impeller, which is arranged in the intake system, of a supercharger or of an exhaust-gas turbocharger. The latter effect is associated with a reduction in efficiency of the compressor and may degrade the impeller blades.
With regard to the issue of the above-described condensate formation, too, an EGR cooler may be expedient or helpful. The cooling of the exhaust gas for recirculation during the course of the recirculation has the advantageous effect that the condensate does not form for the first time in the intake system but forms already during the recirculation, and can be separated off during the course of the recirculation.
A disadvantage of the EGR coolers according to the prior art is that, owing to the principle involved, the useful exhaust-gas energy, that is to say the heat that can be extracted from the exhaust gas in the cooler via coolant, is only available and usable when exhaust gas is being recirculated. If the exhaust-gas recirculation arrangement has been deactivated, such that no exhaust gas is being recirculated, the exhaust-gas energy of the hot exhaust gas often remains unutilized. If it were possible to utilize said exhaust-gas energy without restriction, that is to say to recover said exhaust-gas energy in the context of energy recuperation, it would be possible to achieve further efficiency advantages in the internal combustion engine.
The energy of the hot exhaust gas may, for example, be utilized to reduce the friction losses and thus the fuel consumption of the internal combustion engine. Here, rapid warming of the engine oil via exhaust-gas heat, in particular after a cold start, could be expedient. Fast warming of the engine oil during the warm-up phase of the internal combustion engine ensures a correspondingly fast decrease in the viscosity of the oil and thus a reduction in friction and friction losses, in particular in the bearings which are supplied with oil, for example the bearings of the crankshaft.
Here, the oil could for example be actively warmed via a heating device. For this purpose, it is possible in the warm-up phase for a coolant-operated oil cooler to be utilized, contrary to its intended purpose, for warming the oil.
Fast warming of the engine oil in order to reduce friction losses may basically also be promoted via fast heating of the internal combustion engine itself, which in turn is assisted, that is to say forced, by virtue of as little heat as possible being extracted from the internal combustion engine during the warm-up phase.
In this respect, in the case of a liquid-cooled internal combustion engine, it may also be expedient for heat to be supplied to the coolant of the engine cooling arrangement, in particular in the warm-up phase or after a cold start. It would be possible for the exhaust-gas energy to be utilized for warming the coolant of the engine cooling arrangement.
One previous example, shown in German publication DE 10 2008 020 408 A1 describes an internal combustion engine in which the exhaust-gas energy may be used even when no exhaust gas is being recirculated. That is to say, exhaust-gas energy may be used even when no exhaust gas is being taken from the intake system and introduced into the exhaust-gas discharge system. The return line may be connected optionally to the intake system and/or to the exhaust gas discharge system downstream of the EGR cooler using a control valve which also serves as an EGR valve. Even when the exhaust-gas recirculation arrangement is deactivated and no exhaust gas is being recirculated, the exhaust-gas energy from the hot gases may be used for energy recuperation. The recuperated energy is either used to heat the engine oil more quickly after a cold start, and in this way reduce the friction losses, or to heat the vehicle cabin.
It may also be a disadvantage of EGR coolers according to the prior art that the coolers do not have to be configured with regard to effective energy recovery, with the focus rather being on the cooling of the exhaust gas, that is to say the pure cooling effect. Here, the cooler may be able to cope with all exhaust-gas flow rates for recirculation via the exhaust-gas recirculation arrangement during the operation of the internal combustion engine. In particular, the cooler may be configured to provide cooling to a maximum exhaust-gas flow rate for recirculation. The range of variation of the exhaust-gas flow rate for recirculation via the exhaust-gas recirculation arrangement leads to widely varying pressure conditions at the cooler. The pressure gradient across the cooler changes noticeably in a manner dependent on the exhaust-gas flow rate for recirculation, that is to say in such a relevant manner that it may be taken into consideration in the control or setting of the recirculation rate. The resulting interaction leads to certain dynamics, and demands correspondingly complex or intricate control of the exhaust-gas recirculation arrangement.
The inventors herein have recognized the potential issues with such systems and have come up with a way to at least partially solve them. In one example, the issues described above may be addressed by a method comprising flowing exhaust gas heated via an EGR cooler arranged along a recirculation line to an intake system during an engine deactivation and heating the exhaust gas via an EGR cooler. In this way, EGR may flow to the engine during an engine deactivation even if an EGR request is absent.
As one example, by intrusively flowing EGR during the engine deactivation, the EGR cooler may warm up the EGR via a phase-change material. Heat may be recuperated from exhaust gas during combusting conditions of the engine, wherein the heat may be released to the EGR during the engine deactivation if desired. By doing this, an engine temperature may be maintained, which may decrease frictional losses and increase fuel economy.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.